1. Field of the Invention
The present invention relates to the field of power transmission for use in mechanized agricultural equipment, and more particularly to a high efficiency drive train incorporating an internal brake to prevent freewheeling.
2. Description of the Prior Art
Center pivot and linear irrigation systems are well known in the art for their ability to irrigate large sections of land. Typically, such systems are capable of watering a quarter section of land, i.e., 160 acres, or more. A center pivot irrigation system generally comprises an elongated primary irrigation pipe that extends radially outward from a center pivot. When activated, the irrigation pipe rotates around the pivot, thereby watering the area that the irrigation system passes over and resulting in a circular pattern of water coverage about the central pivot point. The length of time involved in a 360 degree rotation of the irrigation pipe may be up to several days. Likewise, linear systems are comprised of similar irrigation pipes, but move linearly across sections of land to be irrigated.
In conventional irrigation systems, the elongated irrigation pipe or span is supported at spaced apart intervals by a plurality of wheeled towers. Extending between each set of adjacent towers is a truss arrangement utilized to support the span and the water deployment system. Sprinklers are located at spaced intervals along the length of the span or a parallel water conduit. The wheels of each tower are normally positioned perpendicular to the span to permit the tower to follow a prescribed path, either circular for center pivot systems or linear for linear systems.
Whether center pivot or linear, each tower is typically provided with a drivetrain to distribute motive power to the wheels and operable to move the tower in synchronization with the other towers such that the overall length of the span can be maintained in substantially a straight line as the irrigation system moves through its prescribed path. In most conventional systems, the drivetrain consists of a motor, a divider gearbox, at least one drive shaft, at least one wheeldrive gearbox and at least one wheel hub. More specifically, either an electric or hydraulic motor, referred to as a center drive or drive gear motor, is coupled to a divider gearbox centrally located along the base of the tower. The divider gearbox is used to reduce the power input from the motor and divide the power output for transmission to the powered wheel hubs typically positioned at the outer edge of the tower's base. Each wheel hub is attached to a wheel drive gearbox and is driven by a driveshaft extending from the power output shaft of the divider gearbox. Since such irrigation systems may take several days to complete a single watering cycle rotation, the output revolutions per minute of the center drive motors and drive shafts are geared to be very low, generally in the range of 28–86 rpm.
Turning to the divider gearbox and motor assembly centrally located on each tower, the traditional systems of the prior art typically incorporate worm gears as the gearing configuration to transfer power from the motor to the divider gearbox. Specifically, a vertical center motor drive is attached to a worm gear set in which a worm thread on a shaft engages a worm gear. Worm gear boxes are often desirable in applications such as agricultural irrigation systems where the output revolutions per minute are required to be very low and necessitate a large gear reduction.
Notwithstanding the foregoing, it is common that irrigation systems are often used on uneven, sloping or even hilly ground. To the extent such equipment is positioned on an incline, there is a concern that the machinery could roll uncontrolled down the incline as the output shaft “back drives”, i.e., rolling backwards down a hill, or “forward drives”, i.e., rolling forwards down a hill, the gearing. Such uncontrolled motion can result in damage to the drivetrain and the irrigation equipment itself. For this reason, it has also been desirable heretofore for prior art irrigation system drivetrains to incorporate worm gears for their braking characteristics. It is well known in the art that one characteristic of a worm gear set is that the threaded shaft can easily turn the worm gear, but because of the helix angle of the threads on the shaft, friction between the shaft threads and the worm gear prevents the worm gear from turning, i.e., back driving or forward driving, the shaft. This locking feature can act as a brake for the drivetrain when the motor is not operating. Thus, prior art drivetrains for irrigation systems have utilized worm gears to prevent backdriving or forward driving of the drivetrain on an incline.
For the reasons set forth above, worm gears and gearsets are an integral part of the irrigation system drivetrains of the prior art. However, one drawback to standard worm gearsets is that they typically have efficiencies of only about 50%. Specifically, the efficiency of a gearset is measured in part by the input power lost at output by the gearset due to friction. Because worm gears are a friction gear mesh, the friction between the worm gear set results in elevated heat and loss of power, thereby decreasing efficiency.
Notwithstanding the foregoing, electromechanical brakes have also been utilized in the prior art in an attempt to prevent backdriving and forward driving. Because of the electrical components incorporated in such a brake, these brakes are generally enclosed. However, the extreme weather conditions of cold, heat and precipitation often result in a buildup of condensation within even an enclosed case. Because of this environment, the brake components have a tendency to rust. This becomes a particular problem when the machinery has been idle for a period of time, such as during the winter season. In such cases, the brake components often rust bond together, rendering the brake inoperable. In addition to any resultant rusting to the brake components, this moisture can cause malfunction of the electrical components of the brake. Thus, such electromechanical brakes have been found to be undesirable for use in the drivetrain of agricultural irrigation equipment.
Therefore, it would be desirable to provide a high efficiency drive train for agricultural irrigation equipment. The drive train should include a braking mechanism to inhibit freewheeling from the higher efficiency characteristics of the drivetrain. The braking mechanism is preferably mechanically operable and minimizes the likelihood that condensation or corrosion could damage the mechanism.